The functions of the bronchiolar Clara cell are not known although it is generally believed that the cell is secretory. Using a model system, developed in this laboratory, we have identified a low molecular weight protein (Mr 12,500) as the major protein secreted by Clara cells. We have also identified the major secretory protein in pulmonary lavage effluents from the lungs of rabbits and developed a simple procedure for its isolation. Antiserum developed in goats against highly purified Clara cells (purity greater than 90%) has been used to localize Clara cell secretory proteins (CCSP) within the osmiophilic cytoplasmic granules of bronchiolar Clara cells indicating the storage nature of the granules and that secretion of the low molecular weight protein should occur primarily through a regulated pathway. However, we have been unable to demonstrate the regulated pathway and we now conclude that most secretion occurs via a constitutive pathway not involving the secretory granules at all. Using antiserum to CCSP we have demonstrated the presence of an immunogenically similar protein in uterine secretions from pseudopregnant rabbits. We have also isolated uteroglobin from uterine secretions, and with antibodies demonstrated that CCSP and uteroglobin are immunochemically similar proteins. These preliminary data indicate that the major secretory protein of the bronchiolar Clara cells of the rabbit may be uteroglobin. Future studies will seek to establish the relationship of CCSP to uteroglobin and focus on the extracellular function of these low molecular weight secretory proteins.